I think there's no proper to learn something, but I find useful to have several resources available. For example it could be interesting to read the source code of jQuery (or something smaller such as
http://zeptojs.com/ or
https://lodash.com/) and to read a book at the same time (
http://eloquentjavascript.net/) and why not trying to make parallel between them. Also don't hesitate to google stackoverflow for javascript question as it is likely that you'll found any answer to your questions.
To finish I think having friends or a mentor will help you a lot.